Js for、forEach、forof、 forin 四种效率

forin 是遍历对象的 其实不适用遍历数组

let arr=[]
    for (let index = 0; index < 50000000; index++) {
        arr.push(index)
    }
    console.time("for")
    let sum =0;
    for (let index = 0; index < arr.length; index++) {
        sum+=arr[index]
    }
    console.timeEnd("for")     // for: 773.305908203125 ms

    console.time("forEach")
    let sum_1 =0;
    arr.forEach(element => {
        sum_1+=element
    });
    console.timeEnd("forEach") // forEach: 1507.010009765625 ms

    console.time("for of") 
    let sum_2 =0;
    for (const item of arr) {
        sum_2+=item
    }
    console.timeEnd("for of") // for of: 1532.35205078125 ms

    console.time("for in") 
    let sum_3 =0;
    for (const item in arr) {
        sum_3+=item
    }
    console.timeEnd("for in") // for in: 46136.0087890625 ms